A standard lost-grain detector is constituted basically as a transducer which is associated with an impact plate placed along the path of the hay or straw exiting the harvester. This transducer emits electrical outputs when grain seeds strike the plate, as such are very hard as compared with the relatively soft hay, straw, or other stem crop normally passing over the impact plate. The electrical output of the transducer is analyzed and displayed, and also normally compared to a set point corresponding to a maximum loss level. This allows the harvester to be adjusted for maximum efficiency with minimal grain loss, as the harvester is not supposed to eject a significant amount of valuable grain with the stems of the crop.
Thus as described in German patent document 2,448,745 the impact plate is provided on its back face turned away from the outgoing stream of crop with a microphone constituting the transducer. This microphone comprises, as is standard, a casing having a front side that can constitute a diaphragm, and a mechanical/electrical transducing element between the back side of the casing and the diaphragm. In order to maximize the transmission of vibrations from the diaphragm to the transducing element, it is standard to mount a solid peg between the diaphragm of the microphone and the transducer element itself.
Thus a shock delivered by a seed to the impact plate must be transmitted through three interfaces to the diaphragm, that is first from the plate to the diaphragm, then from the diaphragm to the front end of the sound-transmitting peg, and finally from the rear end of the sound-transmitting peg to the transducer itself. Each of these interfaces is typically a glue joint which has some elasticity and which can have different vibration-transmitting properties across a production run of such devices. The diaphragm/impact-plate interface is particularly subject to fouling by corrosion or dirt. Thus there will be considerable losses in this vibration-transmission path, and the response of one such grain-loss detector will vary substantially from that of another due to the differences in mechanical couplings at the three interfaces.